With magnetic recording media having insufficiently improved lubricity, the surface of a magnetic layer is smoothed by repeated contact with contacting parts of the magnetic recording/reproducing deck, resulting in an increase in the coefficient of friction of the magnetic layer. When a magnetic recording medium with a magnetic layer having such an increased coefficient of friction is passed through the magnetic recording/reproducing deck particularly under high humidity conditions (relative humidity; at least 60%), a cohesion phenomenon occurs at running-contact areas of the magnetic recording medium due to the presence of fine water droplets.
When the cohesion phenomenon occurs, if the running tension is greater than the cohesion strength, a stick-slip running is caused, producing a running noise comprising audible sound. In the recording of a magnetic recording medium which is undergoing stick-slip running, the frequency of the recording signal is modulated in the running direction and, therefore, the normal reproduction of records becomes difficult. On the other hand, when the stick-slip running occurs at the time of reproducing a magnetic recording medium which has been recorded, no normal reproduction can be conducted and wow-flutter and jitter phenomena, etc. are caused.
When cohesion is caused between the magnetic recording medium and the running-contacting parts of the magnetic recording/reproducing deck, if the cohesive strength is greater than the running tension, the running of the magnetic recording medium is stopped and reproduction becomes impossible.
The foregoing phenomena are generally called "tape squeal" in the art, and magnetic recording media and magnetic recording/reproducing decks causing such phenomena are seriously reduced in their product value.
The "tape squeal" is caused by the material of the magnetic recording medium running system in the magnetic recording/reproducing deck, the running tension, the running speed, etc. In the case of the magnetic recording medium, the smoothness of the surface, an increase in the coefficient of friction, etc. are responsible for the "tape squeal".
In order to improve the surface lubricity of magnetic recording media, a number of solid or liquid lubricants have been used, as described in U.S. Pat. Nos. 3,476,596, 3,470,021, 3,634,253, 3,630,772, 3,547,693, 3,274,111, 4,018,968, 3,387,993 and 3,492,235. Examples of such lubricants include inorganic or organic fine powders (e.g., Al.sub.2 O.sub.3, graphite, silica, Cr.sub.2 O.sub.3, ZnO, and carbon black) and organic surface active agents (e.g., higher hydrocarbon compounds, aliphatic alcohols, fatty acids, fatty acid esters, fatty acid amides, fatty acid salts, and fatty acid quaternary salts). With regard to the organic surface active agents, there is no limitation on the number of carbon atoms, and those compounds having a boiling point of at least about 100.degree. C. and a melting point of about 150.degree. C. or less are preferred. These lubricants are added in an amount ranging between 0.1 and 20 parts by weight per 100 parts by weight of the binder.
However, the addition of such additives does not always succeeds in preparing a magnetic recording layer having desirable characteristics. For example, the addition of large amounts of additives may result in a decrease in the mechanical strength of the recording layer. In some cases, there is observed the phenomenon that, after the formation of the recording layer, the additive gradually comes out to the surface of the recording layer. Furthermore, the dispersion of magnetic substances is not always satisfactory.
Fatty acid are typical examples of such lubricants (U.S. Pat. Nos. 3,470,021 and 3,634,253). When, however, these fatty acids are added in effective amounts to prevent the "tape squeal", electromagnetic properties are adversely influenced, resulting in a decrease in S/N. Other typical lubricants are fatty acid esters exemplified by butyl palmitate (U.S. Pat. No. 3,274,111). When butyl palmitate is added to the magnetic layer in effective amounts to improve the running properties on guide poles of stainless steel in the running system of a deck for magnetic materials, the "tape squeal" phenomenon occurs at the deck head portion.